Method and apparatus for producing a directable current-conducting gas jet for use in a method for inspecting and measuring nonconductive film coatings on conductive substrates

ABSTRACT

A method and apparatus for producing a directable, electrically conducting gas jet along a path across a test zone, passing an electric current along the jet paths and detecting variations in the current flow in the gas jet stream as a function of the presence of anomalies in the path or changes in the path dimensions. Such anomalies include, but are not limited to, insulators, conductors and semiconductors, gases diffusing into the stream, particulate solids introduced into the stream, electric fields parallel and traverse to the stream and variations in bias potential.

United States Patent (Irishman 51 Feb. 1, 1972 [54] METHOD AND APPARATUS FOR 3,022,430 2/1962 Brown ..324/33 UX PRODUCING A DIRE TABL 3,090,112 5 1963 Smith ....324/33 x CURRENT CONDUCTING GAS JET FOR 3,036,478 7/1963 Brown ..324/33 X USE IN A METHOD FOR INSPECTING 35%? 3/323 3332;113:1111'""""'" 3313123223 AND MEASURING NONCONDUCTIVE 3,449,667 6/1969 0011161116.... .324/33 x FILM COATINGS 0N CONDUCTIVE 2,906,858 9/1959 Morton ..219/121 P sUBsTRA Es 3,317,704 5/1967 Browning ...219/121 P x 3,366,772 l/l968 Wickham et al ..219/121 P [72] lnventor: Charles Richard Cushman, Broomfield,

Colo.

[73] Assignee: Autometrics Co., Boulder, Colo.

[22] Filed: Nov. 12, 1968 [21] Appl. No.: 774,758

[52] US. Cl ..324/33, 324/54 [51] Int. Cl. ..G01n 27/62 [58] FieldofSearch ..324/33,54;219/l21 P; 315/111 [56] References Cited UNITED STATES PATENTS 2,880,373 3/1959 Soloway ..324/33 UX Primary Examiner-Gerard R. Strecker Attorney-Anderson, Spangler & Wymore [57] ABSTRACT A method and apparatus for producing a directable, electrically conducting gas jet along a path across a test zone, passing an electric current along the jet paths and detecting variations in the current flow in the gas jet stream as a function of the presence of anomalies in the path or changes in the path dimensions. Such anomalies include, but are not limited to, insulators, conductors and semiconductors, gases diffusing into the stream, particulate solids introduced into the stream, electric fields parallel and traverse to the stream and variations in bias potential.

14 Claims, 9 Drawing Figures PATENTEU FEB I 1972 SHEEY 1 BF 4 FIG. 2.

INVENTOR. Charles Richard Cushmm II III ATTORNEYS BY F I G. 1. $1M, 6 41 19 PMEWEU FEB H972 $639,831

snm a or a INVENTOR.

Char/es Richard Cushman ATTORNEYS FIG.3.

mmmm H972 $639,831

SHEU 3 0F 4 INVENTOR.

Charles Richard Cushmun BY AT TORN E YS mama] Fm H912 Y 33539331 SHEET M [IF A FIG. 7

1% w m h 306 30? 2 fi 30o fi'llllF' FIG 8 3/6 3/1' 3/8 fiv sue-z 3,5 57 3/6 3/4 33 30a IN V EN TOR. 330 Charles Richard Cushman illll i v BY ATTORNEYS METHOD AND APPARATUS FOR PRODUCING A DIRECTABLE CURRENT-CONDUCTING GAS .IET FOR USE IN A METHOD FOR INSPECTING AND MEASURING NONCONDUCTIVE FILM COATINGS ON CONDUCTIVE SUBSTRATES A flowing, ionizable gas is rendered electrically conductive in an ionizing generator, and is directed from the generator as a jet. A cathode, internal or external to the generator, is included, in contact with the gas.

The ionized gas stream issues from the generator along a path and is intercepted by an anode positioned in the jet path. The anode is biased with respect to the cathode by an external source of e.m.f. at a potential sufficient to create a significant current flow between cathode and anode which is proportional to the conductance of the gas stream between the anode and the cathode.

Determination of thickness and electrical parameters of thin insulating and semiconducting materials deposited on a conducting substrate usually involves the use of a conductive probe on the exposed surface. Knowledge of the area of the probe and some of the parameters of the material permit measurement of other properties of the material. However, degree of contact, air gaps and other boundary uncertainties make the measurement difficult. If an intermediate conductive liquid is used between probe and surface, or if the surface is plated, or a conductive film is vapor deposited on the surface, the test is usually a destructive one, impractical for use at high speed or leaves an undesirable residue.

Use of a conductive gas probe in these measurements greatly facilitates nondestructive, rapid determination of the film properties of the material during passage of the material by the transducer or vice versa. Intimate surface contact with the jet is a surety.

The present invention is directed to a method and apparatus for producing a directable, electrically conductive gas column which can be biased to carry an electrical current and wherein the amount of current carried by the gas column can be used as an indication of conditions existing in the column or at the column boundaries.

The present invention involves the ionization of flowing gas particles in a generator or ionization chamber. The ionization method may be of the electrical discharge type as in DC plasma generators or in gaseous illuminated tubes, the thermal type such as produced by a chemical flame or an electric arc in high-pressure gas, or the radiation absorption type mechanism such as used in ultraviolet ozone generators or in optically pumped lasers.

In contact with the ionized gas is one of a variety of cathodes; for example, the directly or indirectly electrically heated thermionic type common in high-vacuum electron tubes and the cold" secondarily emitting types associated with gaseous luminous tubes. The ionized gas is then directed as a jet across a test zone and impinged on a target anode, having a DC potential with respect to cathode impressed therebetween, to establish a current flow therebetween pro portional to the conductivity of the gas jet. Since they are easily ionizable, inert monoatomic gases such as argon and neon gas or mixtures thereof are preferred for use in the present invention; however, economical compromises may lead to the use of less costly gases such as nitrogen. Any ionizable gas with a relatively low deionization rate is a potential candidate for use with the apparatus.

One embodiment of the present invention involves the energization r ionization of gas particles by passing the gas between a pair of electrodes which have a DC potential established therebetween to produce a glow discharge in the gas. The energized or activated gas particles are then directed as a jet to, and impinged on, a target electrode having a DC potential impressed therebetween and the pair of electrodes to establish a current flow therebetween which is proportional to the conductivity of the gas jet.

The gas is ionized according to the preferred embodiment of the present invention by passing the gas through an ionizing generator containing a thermionic cathode and directing the ionized gas stream in a jet away from the generator across a test zone to a target anode within a period of time such that the ionization or other electrical conductivity properties of the gas are not significantly diminished or decayed on reaching a target anode, which is electrically biased with respect to the cathode,"to establish and conduct a modulatable and measurable amount of current flow.

The apparatus, according to the present invention, is arranged to provide for passage of the gas through an ionizing generator containing a cathode and todirect the gas stream as a jet across a test zone to impinge upon a target anode which is maintained at a bias potential with respect to the cathode such as to cause an electrical current to flow between anode and cathode'via the gas jet across the test zone. The ionized gas is directed from the ionizing generator to the anode at such a velocity as to minimize the time-related decay of ionization and to maximize the current-carrying capacity and attendant sensitivity of the jet to anomalies positioned in the path thereof.

The present invention, by virtue of its use of a moving gas as a medium for conveying electrical current, has potentially important uses in the field of nondestructive testing in which a contactless probe and/or an easily accessible gaseous measurement zone may be used or required.

Thosev skilled in the art will readily recognize the many applicationsof the present invention as a transducer from physical to electrical modulation of an electrical signal. Examples are electrostatic reading and writing on a conductor-backed dielectric storage disc, measurement of the thickness of nonconductive coatings on conductive substrates, detection and measurement of areas of exposed metal in surfaces covered with nonconductive coatings, detection and location of small holes in nonconductive films on a metal backing member, detection of nonconductive contaminants on conductive surfaces and contactless switching.

It is, therefore, an object of the present invention to provide a method and apparatus for activating a gas and directing an activated jet thereof which is electrically biased to produce current flow and measuring the amount of current as a function of the anomalies in the path of the jet.

Another object of the present invention is to provide a method and apparatus for producing a current-carrying gas jet which can be directed to a target biased to produce a current flow and detecting the presence and amount of current flow as an indication of the presence and quantity of material in the path of the jet.

A further object of the invention is to provide an improved gas ion generator operating with DC excitation voltage in the glow discharge region to energize a stream of gas which is directed as a jet therefrom to impinge upon a target electrode biased at an accelerating potential with respect to the generator sufficient to produce significant current flow therebetween.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the drawings:

FIG. 1 is a side elevational view partly in cross section of an apparatus according to the present invention for testing and determination of voids in a plastic coating applied to metal containers;

FIG. 2 is a view along line 2-2 of FIG. 1;

FIG. 3 isa view along line 33 ofFlG. 1;

FIG. 4 is a view along line 4-4 of FIG. 1;

FIG. 5 is a view in cross section to an enlarged scale of the ion generator according to this invention;

FIG. 6 is a view along line 6-6 of FIG. 5;

FIG. 7 is a schematic diagram of the electrical configuration of the power supply and readout circuits of the present invention;

FIG. 8 is a diagrammatic representation of an ionizing generator and circuit according to the present invention of the induction coupled electrodeless type; and,

FIG. 9 is a diagrammatic representation of an ionizing generator and circuit according to the present invention wherein the generator is of the chemical burner electrodeless type.

As referred to in the following disclosure, an ion is a charged atom, molecule or radical whose migration effects the transport of electricity through an electrolyte and to some extent through a gas. The term cathode refers to the electrode through which a current leaves an electrolytic cell, gas discharge or thermionic valve to return to an external source of electromotive force. The term anode refers to the electrode through which a current enters an electrolytic cell, gas discharge or thermionic valve from an external source of e.m.f.

Briefly, the present invention is directed to a method of producing an electric-current-conducting jet of a gas which is impinged upon a target operating at a bias potential with respect to a cathode in the gas jet and measuring the current flow between the origin and the target as being indicative of any anomalies in the path of the gas jet. The generation of the electric-current-conducting jet may be accomplished by passing a gas between a pair of electrodes having a DC electrical potential drop impressed thereacross sufficient to produce a glow discharge, but insufficient to produce an arc, and directing the gas from the glow discharge zone to an external target anode.

The apparatus for performing the present method includes an electrode assembly positioned within a housing through which the gas to be activated passes while a DC potential is applied across the electrodes to produce ionization of the gas. The electrode assembly housing includes an orifice through which the activated gases issue as a jet and impinge on an external anode. The external anode is maintained at a DC bias potential with respect to the electrodes sufficient to create current flow between the cathode and the external anode via the gas jet.

When nitrogen is used as the gas, it is ionized under glow discharge or corona conditions in the absence of any sparking or arcing. A glow discharge or corona condition is described as a discharge which is completely devoid of any random noise, such as sparking and arcing in the discharge.

It is generally an important aspect of the present invention that the unmodulated current flow between the nozzle and the target be substantially constant. The gas jet issuing from the nozzle issues as a current conductor and the amount of current flow will vary from zero or a no current flow to full current flow as a function of anomalies existing or introduced into the normal path of the jet. These anomalies may comprise insulators, conductors and semiconductors, gases diffusing into the stream, particulate solids introduced into the stream, electric fields parallel and traverse to the stream and variations in bias potential. Thus, where the path is blocked or disrupted, there is produced an on or off condition of current flow. Where a partial insulator is introduced into the jet, the modulation of current flow is a function of the thickness or insulation characteristics of the insulator. A modulation of current flow may likewise be occasioned by the entry of another gas or particles of a solid or liquid into the jet path.

Referring now to FIGS. 1-6 of the drawings, there is shown an embodiment utilizing the teaching of the invention. This embodiment is in the form of a system for inspecting plastic film coatings on metals for areas of exposed metal (voids) and for measuring the thickness of such plastic films. In the production of metal containers for products for human consumption, it is usually desirable to line the container with a thin coating or film of a plastic material to prevent the contents from contacting the metal walls of the container and becoming contaminated. It is highly desirable to determine if there are any voids, holes or thin spots in the plastic film likely to make it fail in use. FIGS. 1-6 depict inspection apparatus 16 according to the present invention, which serves as a detector of voids or significant anomalies in the plastic liningof a container. The inspection apparatus comprises a sample support 18 and a scanning head support 20. The sample support 18 comprises a rotatable table 22 mounted for rotary movement in a work surface 24. The sample holder 22 is secured to a block 26 of insulation material by means of screws 28 and the like. Block 26, in turn, is secured to a hollow shaft 30 by means of screw 32. Shaft 30 isjournaled in bearings 34 and 36 located in either end ofa sleeve 38 received in an opening 40 in work surface 24. Sleeve 38 is provided with a lip 42 which engages the top of work surface 24 to support the sleeve 38 and associated components. A pulley 44 is attached to the lower end of shaft 30 and pulley 44 is connected to pulley 46 by means of a belt 48. Pulley 46 is mounted on the shaft of motor 50 which is supported from the work surface 24 by means of bolts 52 and the like and secured by nuts 54. Hollow shaft 30 has another shaft 56 concentrically positioned therein and electrically insulated therefrom by an insulating sleeve 58 at the lower end and passes through an opening 60 in block 26. Shaft 56 is received in an opening 62 in table 22 in electrical continuity therewith and for rotation therewith. A tachometer 64 or other position transducer means is supported from the work surface 24 by means of a bracket 66. Transducer 64 is connected to shaft 56 and is adapted to provide an output indicative of the rotational positioning of table 22. An electrical connection is obtained with shaft 56 by means of a brush 68 which is biased into contact therewith by means of spring 70 secured to bracket 66. Table 22 is provided with a central depression 72. One or more spring-loaded detents 74 are provided in the sidewalls of depression 72 to make electrical contact with and hold a container such as a can 76, shown in dotted lines.

Referring again to FIG. 1 and FIG. 2, the scanning head support 20 is supported for movement on a lead screw 78 mounted for rotation within support column 80. Column 80 is seen as a vertically positioned tube supported and secured to the top of work surface 24 by means of sleeve bracket 82 and set screws 83. Column 80 is received within a sleeve bracket 82 secured to work surface 24 by suitable means, not shown, and passes through an opening 84 therein. The lower end of tube 80 has a bracket 86 secured thereto as by welding at 88. A drive motor 90 is attached to bracket 86 with the drive shaft 91 axially aligned with tube 80. The lower end of lead screw 78 is connected to and supported by the drive shaft 91 of motor 90 by means of coupling 92. The reduced diameter upper end 94 of lead screw 78 is journaled for rotation in a bearing 93 supported in closure cap 96 for tube 80. The end 94 of lead screw 78 projects through bearing 93 and a handwheel 98 is attached thereto by suitable means.

The scanning head support 20 is attached to a projection 99 carried by the lead screw follower assembly 100 and fastened thereto as by means of fasteners 102 and the like. The follower assembly 100 comprises a threaded nut 104 engaging the threads of lead screw 78. The lower portion of nut 104 is bored out to a diameter larger than lead screw 78 and internally threaded as at 106. A tubular sleeve 108 having an internal bore of a larger diameter than lead screw 78 is threadedly received in the internal threads 106 of nut 104. A collar 110 is positioned between nut 104 and shoulder 112 formed by a reduced diameter portion 114 on the upper end of sleeve 108. The collar 110 is provided with a protrusion or peg 116 which is received in a vertically aligned slot 118 in column 80 which also receives projection 99 of the follower assembly 100. As lead screw 78 turns, the nut 104 is prevented from turning via the nut retainer and peg 116 moving in slot 118, causing the nut 104 and the follower assembly 100 to move either up or down, depending on the direction of rotation of the lead screw 78.

Sleeve 108 is provided with an enlarged diameter portion adjacent the lower end forming a shoulder 120. Another sleeve 122 within internal bore 124 is of a size sufficient to receive the outside diameter of sleeve 108 in sliding relation.

The outer diameter of sleeve 122 is of a size to provide a sliding fit with the inside diameter of column 88. The upper portion of sleeve 122 is provided with a section 126 of reduced outside diameter and defines a shoulder 128 with the lower portion thereof. The upper portion section 126 is provided with a vertically elongated slot 130 inclined slightly from the vertical and through which a peg 132 attached to sleeve 108 projects. A compression spring 134 is positioned about sleeve 108 and the reduced diameter portion 126 of sleeve 122 to provide a spring bias between the lower surface of collar 110 and the shoulder 128 of sleeve 122 and holds peg 132 at the upper end of slot 130. Spring 134, in addition, is attached to and provides a torsional bias between collar 110 and sleeve 122 to bias projection 99 into intimate contact with one side of slot 118 and pin 132 against one side of slot 130. A travel stop 136 for the follower assembly is secured to tube 80 by means of fastener 138 or the like and the stop can be vertically positioned as desired to limit the extent of downward travel of the follower assembly. The stop is in the shape of a split annulus for the reasons to be stated. Suffice it to say that, when the follower engages the stop, further downward movement thereof is prevented; however, the central opening in the stop 136 is of a size sufficient to permit the lower end of the sleeve 108 to pass therethrough and sleeve 122 is moved upward against spring 134 causing peg 132 to move downward in slot 130 and rotate sleeve 122 and projection 99 in a horizontal plane as best seen in F IG. 4, as shown in dotted lines.

A further nut 140 is positioned on lead screw 78 below follower assembly 100 for indexing purposes. Nut 140 has a pin or peg 142 attached thereto which is positioned in slot 118 of tube 80 and prevents nut 140 from turning with respect thereto. Nut 140 is also provided with a projection or finger 144 which projects through slot 118. The split 137 of stop 136 is aligned with slot 118 and the diameter of nut 140 is such as to pass through the central opening of stop 136 and split 137 allows pin 142 and finger 144 to pass therethrough as lead screw 78 is rotated.

A limit switch actuating rod 146 is received in and passes through 'a hole in the end of finger 144 and is supported for vertical movement by a U-shaped bracket 148 attached to motor support bracket 86. Rod 146 passes through an opening 150 in work surface 24. Rod 146 may conveniently be threaded and is supported at the lower end by means of a spring 152 positioned between a nut 154 and a leg of bracket 148. A lower or override actuate finger 156 is mounted for sliding movement on rod 146 and is held in place thereon by springs 158 and 160 held between nut 154 and nut 162. The primary motor direction reversing actuate finger 164 is mounted for slidable movement on rod 146 and held in placeby springs 166 and 168 between nuts 170 and 172. Adjacent the upper end of rod 146 and above finger 144, a pair of nuts 174 are locked in place to provide an upper limit stop'for reversing the direction of rotation of the motor. Another pair of nuts 176 are positioned on rod 146 below finger 144 to provide a lower limit stop. A pair of microswitches 178 and 180 are mounted on bracket 148 to be actuated by actuator finger 164 when stops 174 or 176 are engaged by projection 144. Switch 178 is actuated when stop 174 is engaged and switches the power to reverse the direction of rotation of motor 90 and lead screw 78. Switch 180 is actuated when stop 176 is engaged and switches the power to reverse the direction of rotation of motor 90 and lead screw 78. Switches 182 and 184 are provided as override or emergency switches so that, if either switch 178 or 180' fail to reverse the motor direction when either stop 174 or 176 is engaged, on further travel of rod 146, switches 182 and/or 184 will be actuated to brake the motor and remove power from the system.

A gear-driven potentiometer 186 or other suitable linear position transducer is mounted upon bracket 148 as a fixed part of the apparatus. A rack gear 188 is attached to finger 144 for vertical movement therewith. The rack gear 188 engages a pinion gear 190 connected to drive potentiometer 186 such that an output signal is available from a potential applied to the potentiometer which is representative of the vertical positioning of the follower assembly 100. This signal can be interpreted by a readout device to locate the vertical positioning of the follower assembly with respect to the work surface 24.

The scanning head support 20 includes a rectangular tube 192 telescopically receiving projection 99 and is attached thereto and to the follower assembly by fasteners 102. Tube 192 is attached to a vertically positioned tube 194 positioned above rotatable table 22. A gas supply tube 196 is concentrically positioned inside tube 194 and is held in spaced relation thereto by electric insulators 198 and 200. Tube 196 is connected to a gas supply line 202 via manifold 204. Gas supply line 202 is connected to a suitable gas supply, not shown. The upper end of tube 194 is received and secured in an opening in the bottom 206 of an enclosure 208. Wall 210 of enclosure 208 has a pair of insulated through-connectors 212 and 214 with the conductor 216 of connector 214 being electrically connected to the manifold 204 and tube 196. As best seen in FIGS. 1 and 5, a conductor 218 runs the length of the tube 194 and is insulated from tube 194 in passing therethrough by means of insulators 198 and 200. Conductor 218 is connected to conductor 222 passing through insulated connector 212 in wall 210 of enclosure 208.

A scanning head 21 is attached to the lower end of tube 196. The head 21 comprises a hollow cylindrical housing 224 having an opening 226 in the side wall thereof and the tube 196 is attached thereto by suitable means with the opening 226 being in registry with the interior of tube 196 and commu' nicating'therewith. The housing 224 is provided with external threads 228 and 230 at either end. A cylindrical sleeve 232 of an electrical insulating material capable of withstanding substantial amounts of heat, such as a ceramic, is positioned inside housing 224. Sleeve 232 is provided with one or more passageways 234 extending radially from within the central opening 236 through the sidewall of the sleeve. Passageways 234 are positioned to communicate the central opening 236 with opening 226 of housing 224 and with tube 196. A metal nozzle 238 is received inside hosing224 at one end in contact with sleeve 232. Nozzle 238 is provided with a bore 240 axially aligned with housing 224 and sleeve 232. Bore 240 is of a diameter less than that of the central opening 236 of sleeve 232, but is of an enlarged diameter 242 near the outer extremity to provide awindshield effect as will be explained. The outer extremity of nozzle 238 is tapered and a cap 244 having a central opening 246 therein is threadably attached by means of threads 228 to housing 224. In the end of the housing 224 opposite nozzle 238 is fitted an insulator 248. insulator 248 is provided with a central opening 249 therein and a centrally positioned radially extending flange 250 of a diameter greater than the internal diameter of housing 224. The flange 250 forms a shoulder 251 which abuts the end of sleeve 224. The end of insulator 248 toward the nozzle is provided with a reduced diameter portion 252 which is received within an enlarged diameter'253 of bore 236 of sleeve 232. An electrode 254 is secured in an electrode holder 255, which holder is threaded throughout its length. The electrode holder is threadedly received within an internally threaded sleeve 256 having a centrally positioned radially extending flange 257. Flange 257 is positioned and held between the shoulder formed between the enlarged diameter 253 of bore 236 and the reduced diameter portion 252 of insulator 248. An end cap 258 having a central opening 215 is threadedly received on housing 224 by means of threads 230 and the opening 215 is of a diameter to receive insulator 248 and secure flange 250 of insulator 248 between end cap 258 and the end of housing 224. A nut 213 is threadedly received on electrode holder 255 and is received within a bore 223 in insulator 248. An electrical contact sleeve 221 is slipped over the outer end of electrode holder 255 and clamped between nut 213 and nut 219 threadedly received on electrode holder 255. Electrode holder 255 is provided with a screw slot 217 on the outer end for adjustably positioning the electrode holder 255 and electrode 254 axially within threaded sleeve 256 and sleeve 232 with respect to nozzle 238. The end of conductor 218 passing through insulator 200 is connected to electrical contact sleeve 221 to electrically connect with electrode holder 255 and electrode 254. Referring again to nozzle 238, the enlarged diameter portion 242 of bore 240 serves to protect the gas jet issuing from bore 240 from the influence of air currents set up by relative movement of the head 21 and the surfaces being examined.

Referring now to FIG. 7, there is shown the electrical circuit diagram of the inspection apparatus according to the present invention. The power supply 259 for creating the ionization in generator 21 is shown as a DC source and in the particular generator design produced a potential of about 2,000 v. DC. The power supply is seen to comprise a suitable transformer 260 having a secondary winding to transform line voltage to the higher voltage. A pair of diodes 262 and 264 are connected to the secondary winding as a full-wave rectifier. The

center-tap 266 of the secondary is left floating and is connected to one side of a pi filter network 268. The output of the full-wave rectifier is connected to the other side of the filter network 268. The output of the rectifier which is at a negative potential is connected to center electrode 254 via ballast resistor 270. The center-tap 266, which is the positive side of the power supply, is electrically connected to nozzle 238 of generator 21. A can 76 whose coating is to be tested is shown connected to a positive ground potential. Power supply 259 with ballast resistor 270 serves to create a DC current between center electrode 254 and nozzle 238 sufficient to create and maintain glow discharge or ionization therebetween devoid of any noise or sparking. With the present design, a generator current flow on the order of about to 40 milliamperes is sufficient to maintain the ionization under these conditions. An accelerating bias is maintained between generator 21 and can 76 which serves as the target for generator 21 and the impinging ionized gas. The bias voltage is produced by another transformer 272 having a center-tap secondary winding connected to a pair of diodes 274, 276 as a full-wave rectifier, the output of which is negative. The output of the rectifier is connected across a pi filter 278 for filtering and across a potentiometer 280, the sliding tap of which is connected to the center-tap 266 of transformer 260 which is connected to target 76 through a resistor 282. An amplifier 284 is connected across the resistor 282 and the output from amplifier 284 is proportional to the current flow between generator 21 and target 76. With a suitable potential of about 20-40 volts DC bias between generator 21 and target anode 76, a current flow on the order of about ISO-250 microamperes is obtained.

A variety of bridge-type instruments could be substituted for the resistance 282 and amplifier 284 combination. One such instrument is a General Radio 650-A impedance bridge, manufactured by General Radio Co. When the bridge is so connected, the following relation can be made use of to measure capacitance when the surface is an insulator.

C measured capacitance (Farads) e, relative dielectric constant of insulating wall e,,= permativity of free space (Farads/M) A effectivejet area (M 1, thickness offilm (M) Therefore, knowledge of the film thickness permits calculation of the relative dielectric constant of the film and vice ver- Where the instrument according to this invention is to be used in the determination of coating thickness in coated metal containers and the gas jet is scanning the film surface in the manner as set forth in FIG. 1, the scanning head progresses along the internal surface of the can describing a spiral path with a pitch fine enough to assume the overlap of adjacent scans. A solution of a specific differential equation for this mode ofoperation as depicted by the circuit ofFlG. 7 is:

lil'is l 6 I) Where R,=jet resistance and external circuit resistance (Ohms) T/A scanning time per unit area of film (Seconds/M l,= film thickness (M) 6, relative dielectric constant of film D constant I current (amperes) The creation of the ionization may be by electrode discharge, electrodeless discharge or by a chemical reaction such as a flame; however, the generation must be accomplished in such a manner that the plasma is completely devoid of any sparking noise normally associated with a corona discharge at high pressures. In the embodiment of the ionization generator 21, a gas such as nitrogen, argon and the like is introduced into bore 236 via bore 226 and pipe 196. The major part of gas entering generator 21 by pipe 196 flows through passageways 234 and out the opening 240 in nozzle 238. A portion of the gas flows at low velocity around and past center electrode 254 and an electrical discharge is produced in the gas between center electrode 254 and nozzle 238. The gas issuing from nozzle 238 carries part of the resulting plasma through the nozzle and is impinged on an external anode 76. All of the discharge components cannot be conveyed to the external anode as enough of the ions and electrons must remain in the generator to sustain the continued discharge. The ionization generator of the present invention includes an emitting thermionic cathode which can be heated either by the plasma, induction, a combination of both or resistively heated. Cathode temperatures on the order of l,5002,000 C. are necessary for a significant current.

The application of a potential on the order of 20 to 40 volts between the nlasma source or generator 21 as the cathode and target 76 as the anode will cause space charge modulation near the cathode and electron conduction through the gas to the external anode. The external current through the gas will be modulated by interfering dielectrics positioned in the gas path; by variations in the spacing of the generator nozzle and anode; by influences on the external beam such as particulate solids (dust, smoke, etc.) between the nozzle and external anode; by electric fields produced by grids and the like between the nozzle and the external anode; and, by variations in effective anode area. The modulation of the external beam is detected and amplified by amplifier 284 for readout. The output of amplifier 284 is proportional to the beam current flow which is representative of the modulation of the beam.

generator is seen to consist of a ceramic tube 286 having an inlet 288 at one end and a nozzle opening 290 of reduced diameter at the other end. Intermediate the ends, one or more gas inlets 292 are positioned to permit gas introduced thereinto to join gas introduced into inlet 288 and issue through nozzle opening 290. A cathode 294 is positioned inside tube 286. An induction coil 296 is wound around tube 286 to encompass a portion of cathode 294. A radiofrequency power source 298 is connected to coil 296 and applies energy thereto to heat cathode 294. A DC power source 300 is connected between cathode 294 and an external anode 302 via resistor 304. An amplifier 306 is connected across resistor 304. Due to thermionic emission from cathode 294 resulting from the heating thereof by induction coil 296, the introduction of gas into inlet 288 creates ionization in tube 286. The gas introduced into inlets 292 at relatively high velocity carries ionized gas and impinges same on anode 302 and current flow is occasioned between cathode 294 and anode 302. This current flow is reflected as a voltage drop across resistor 304 which is amplified by amplifier 306. The use of an electrodeless ionization generator has certainadvantages since electrodes are the greatest source of ion recombination.

Referring now to FIG. 9, there is shown in diagrammatic representation an electrodeless ionization generator of the chemical burner type. The generator is seen to consist of a ceramic tube 308 having an inlet 310 at one end and a nozzle opening 312 of reduced diameter at the other end. Intermediate the ends, one or more gas inlets 314 are provided adjacent the nozzle opening 312 for the introduction of relatively high-velocity gas to be energized such as argon and the like. Inlets 316 are also provided adjacent opening 310 for the introduction of gases which will chemically react to create a plasma or burner effect, such as oxygen and hydrogen and the like. Inlets 316 are positioned such that the burning reaction occurs axially of tube 308. A pointed cathode electrode 318 is positioned concentrically within tube 308 with the point thereof positioned within the plasma produced by reacting gases entering inlets 316. A DC power source 320 is connected between electrode 318 and an external anode 322 via resistor 324. An amplifier 326 is connected across resistor 324 to detect and amplify signals representative of current conduction by activated gas issuing from nozzle opening 312. The heating of electrode 318 by the exothermic reaction of gases introduced into the tube via inlets 316 produces thermionic emission from electrode 318. The gases introduced into the tube via inlets 314 carry ionized gas out nozzle opening 312 to impinge on target 322 establishing a current flow from source 320 to electrode 318 via the ionized gas to target 322 through resistor 324 and back to the source. It will be understood that the oxygen and hydrogen gases are only representative of the gases which might be introduced into inlets 316. Other gases which would chemically react with an exothermic reaction to provide the necessary heating could be used.

What is claimed is:

1. Apparatus for detecting and measuring a quantity of material in a predetermined path comprising:

an enclosure having an inlet for the introduction of a gas to be ionized and an outlet for the discharge of an ionized ionizing means within said enclosure including an electrode for ionizing a gas entering said inlet and exiting said outlet;

target electrode means spaced from said enclosure and positioned in the path of gas issuing from said outlet to define a material-receiving space therebetween;

a source of electrical potential directly connected between said electrode of said ionizing means and said target electrode whereby an electrical bias and current flow is established between said ionizer electrode and said target electrode of a magnitude insufficient to support an electric arc therebetween; and

means for measuring the current flow as a function of the material in said space.

2. The apparatus of claim 1 wherein the target electrode is biased at a positive potential with respect to the ionizer electrode.

3. The apparatus of claim 1 wherein the electrode of the ionizing means comprises a heated electrode adapted to thermally ionize the gas particles passing through the enclosure and includes means for heating same.

4. The apparatus of claim 3 wherein the means for heating said ionizer electrode includes another electrode, and a source of electrical current connected between said electrodes and adapted to produce a glow discharge in the gas within the enclosure to heat said ionizer electrode.

5. The apparatus of claim 3 wherein the means for heating said ionizer electrode includes burner means for the introduction of at least one other gas adapted to heat said electrode by exothermic chemical reaction.

6. The apparatus of claim 3 wherein the means for heating said ionizer electrode comprises induction heating means positioned and adapted to inductively couple a source of alternating current to said electrode.

7. A method for detecting and measuring a quantity of material in a predetermined path which comprises the steps of:

introducing a gas into a chamber containing an electrode to exittherefromasajet; Ionizing. said gas within said chamber prior to its exit therefrom;

positioning a target electrode in the path of the gas jet exiting from said chamber and spaced therefrom;

directly connecting a source of electrical potential between the electrode in said chamber and said target electrode of sufficient potential to establish a significant current flow therebetween, but insufficient to support an electric arc; and

positioning a material within the space between 'said chamber and said target electrode in the path of the gas jet; and, measuring the current flow between said chamber and said target as a function of the material in the space.

8. A method according to claim 7 wherein the ionization of the gas comprises the step of heating the electrode in said chamber by producing an electrical glow discharge within the chamber.

9. A method according to claim 7 wherein the ionization of the gas comprises the step of heating the electrode in said chamber by introducing and reacting exothermically chemical reactive gases in the chamber.

10. A method according to claim 7 wherein the ionization of the gas comprises the step of heating the electrode in said chamber by inductively coupling a source of alternating current thereto.

11. Apparatus for detecting and measuring a quantity of material in a predetermined path comprising:

gas ionizer means adapted to ionize a gas therein and having at least one electrode;

means adapted to direct ionized gas from said ionizer as a gas jet;

a target electrode spaced from said ionizer and positioned in the path of the gas jet; to define a material receiving space therebetween;

a source of electrical potential directly connected between said target electrode and an electrode of said ionizer adapted to establish an electrical bias and flow of current therebetween of a magnitude insufficient to support an electric arc; and

means for measuring the current flow as a function of the material in said space.

12. Apparatus according to claim 11 wherein the gas ionizer comprises an enclosure having an inlet to admit a gas thereinto to be ionized, a pair of electrodes, means to supply power to the electrodes to activate a glow discharge between the electrodes to ionize the gas and outlet means adapted to direct the ionized gas as a jet to impinge on said target electrode.

13. Apparatus according to claim 11 wherein the gas ionizer comprises an enclosure having an inlet to admit a gas thereinto to be ionized, an induction coil surrounding said enclosure and said electrode, a source of alternating current connected to said coil inductively coupled to said electrode to heat same to ionize the gas and outlet means adapted to direct the ionized gas as a jet to impinge on said target electrode.

14. Apparatus according to claim 11 wherein the gas ionizer comprises an enclosure having an inlet to admit a gas thereinto to be ionized, at least one additional inlet for a reactant gas adapted to produce an exothermic reaction and arranged to impinge upon and heat said electrode to ionize the gas and outlet means adapted to direct the ionized gas as a jet to impinge on said target electrode. 

1. Apparatus for detecting and measuring a quantity of material in a predetermined path comprising: an enclosure having an inlet for the introduction of a gas to be ionized and an outlet for the discharge of an ionized gas; ionizing means within said enclosure including an electrode for ionizing a gas entering said inlet and exiting said outlet; target electrode means spaced from said enclosure and positioned in the path of gas issuing from said outlet to define a material-receiving space therebetween; a source of electrical potential directly connected between said electrode of said ionizing means and said target electrode whereby an electrical bias and current flow is established between said ionizer electrode and said target electrode of a magnitude insufficient to support an electric arc therebetween; and means for measuring the current flow as a function of the material in said space.
 2. The apparatus of claim 1 wherein the target electrode is biased at a positive potential with respect to the ionizer electrode.
 3. The apparatus of claim 1 wherein the electrode of the ionizing means comprises a heated electrode adapted to thermally ionize the gas particles passing through the enclosure and includes means for heating same.
 4. The apparatus of claim 3 wherein the means for heating said ionizer electrode includes another electrode, and a source of electrical current connected between said electrodes and adapted to produce a glow discharge in the gas within the enclosure to heat said ionizer electrode.
 5. The apparatus of claim 3 wherein the means for heating said ionizer electrode includes burner means for the introduction of at least one other gas adapted to heat said electrode by exothermic chemical reaction.
 6. The apparatus of claim 3 wherein the means for heating said ionizer electrode comprises induction heating means positioned and adapted to inductively couple a source of alternating current to said electrode.
 7. A method for detecting and measuring a quantity of material in a predetermined path which comprises the steps of: introducing a gas into a chamber containing an electrode to exit therefrom as a jet; ionizing said gas within said chamber prior to its exit therefrom; positioning a target electrode in the path of the gas jet exiting from said chamber and spaced therefrom; directly connecting a source of electrical potential between the electrode in said chamber and said target electrode of sufficient potential to establish a significant current flow therebetween, but insufficient to support an electric arc; and positioning a material within the space between said chamber and said target electrode in the path of the gas jet; and, measuring the current flow between said chamber and said target as a function of the material in the space.
 8. A method according to claim 7 wherein the ionization of the gas comprises the step of heating the electrode in said chamber by producing an electrical glow discharge within the chamber.
 9. A method according to claim 7 wherein the ionization of the gas comprises the step of heating the eLectrode in said chamber by introducing and reacting exothermically chemical reactive gases in the chamber.
 10. A method according to claim 7 wherein the ionization of the gas comprises the step of heating the electrode in said chamber by inductively coupling a source of alternating current thereto.
 11. Apparatus for detecting and measuring a quantity of material in a predetermined path comprising: gas ionizer means adapted to ionize a gas therein and having at least one electrode; means adapted to direct ionized gas from said ionizer as a gas jet; a target electrode spaced from said ionizer and positioned in the path of the gas jet; to define a material receiving space therebetween; a source of electrical potential directly connected between said target electrode and an electrode of said ionizer adapted to establish an electrical bias and flow of current therebetween of a magnitude insufficient to support an electric arc; and means for measuring the current flow as a function of the material in said space.
 12. Apparatus according to claim 11 wherein the gas ionizer comprises an enclosure having an inlet to admit a gas thereinto to be ionized, a pair of electrodes, means to supply power to the electrodes to activate a glow discharge between the electrodes to ionize the gas and outlet means adapted to direct the ionized gas as a jet to impinge on said target electrode.
 13. Apparatus according to claim 11 wherein the gas ionizer comprises an enclosure having an inlet to admit a gas thereinto to be ionized, an induction coil surrounding said enclosure and said electrode, a source of alternating current connected to said coil inductively coupled to said electrode to heat same to ionize the gas and outlet means adapted to direct the ionized gas as a jet to impinge on said target electrode.
 14. Apparatus according to claim 11 wherein the gas ionizer comprises an enclosure having an inlet to admit a gas thereinto to be ionized, at least one additional inlet for a reactant gas adapted to produce an exothermic reaction and arranged to impinge upon and heat said electrode to ionize the gas and outlet means adapted to direct the ionized gas as a jet to impinge on said target electrode. 